Furnace Wall Temperature Research Articles

Numerical Investigation of the Influence of Furnace Wall Temperature on Pulverized Coal Combustion and NOx Emissions in a Supercritical CO2 Boiler

A study on the combustion process and NOx emission of pulverized coal in a supercritical CO2 boiler was carried out by using the method of numerical simulation. Using a comprehensive combustion model and a post-treatment NOx formation model, the influence of wall temperature on the characteristics of combustion and NOx emissions of a 300 MW tangential coal-fired boiler were studied. The temperature field and the concentration distribution of waste flue gas at the furnace outlet are analysed and studied. It was found that the furnace wall temperature seriously affected both the distribution of the temperature field and the emission of NOx. The supercritical CO2 boiler’s partial flow strategy led to further increases in temperature and NOx near the tail. According to the results of the calculations, the supercritical CO2 boiler might raise the exhaust gas temperature, making the difficulty of extracting energy from waste gas increased dramatically. The increased NOx emissions will lead to higher cost for implementation of after treatment of NOx emissions. The work’s findings can be used to improve the design of coal-fired supercritical CO2 boilers as well as gain a deeper comprehension of the formation of NOx.

Modelling the Thermal Treatment and Expansion of Mineral Microspheres (Perlite) in Electric Furnace Through Computational Fluid Dynamics (CFD): Effect of Process Conditions and Feed Characteristics

ABSTRACT A model-based investigation of the interplay between furnace setup and spatio-temporal evolution of particles’ state variables is also presented. The effect of feed size distribution and processing of ultrafine ore are discussed. The process is controlled primarily by the wall temperature and secondarily the air feeding rate; an increase of furnace wall temperature from 900 to 1200°C reduced the final density to 484, 979, and 1262 kg/m3 for feed size of 150 μm, 250 μm, and 350 μm. Model predictions are in good agreement with the experimental measurements possessing differences below 10% in most cases.

Reaction zone characteristics, thermal performance and NOx/N2O emissions analyses of ammonia MILD combustion

Pure ammonia MILD combustion is studied numerically in a furnace. The effects of furnace wall temperature and dilution of inlet air with nitrogen on the characteristics of reaction zone (RZ), temperature distribution and exhaust emissions are investigated. The results show that lowering the furnace wall temperature and adding diluent to the inlet air, result in expansion of the RZ, reduction of fuel and oxidizer consumption rates, lower Damkohler, increased turbulent Reynolds and more similar RZ properties to the distributed combustion regime in the combustion regimes diagram. At a lower concentration of oxygen in the oxidizer, a higher threshold wall temperature is required for stable MILD combustion. Reducing the wall temperature and oxygen mole fraction in the oxidizer, increases the uniformity of temperature distribution. Analysis of exhaust emissions shows that reducing wall temperature and oxygen level in the oxidizer stream can significantly reduce NOx pollution, however, N2O emission rises.

PARAMETRIC STUDY OF THE NON-PREMIXED COAL COMBUSTION IN FURNACE FOR HEAT TRANSFER AND EMISSION CHARACTERISTICS

Steady-state turbulent non-premixed combustion of pulverized coal has been modeled in the two-dimensional furnace. Pulverized coal of three different types, low volatiles coal, medium volatile coal and high volatile coal, has been considered. The coal is injected through the center of the furnace and air is being supplied with two inlets (top inlet and bottom inlet) at different velocities. Taking advantage of the symmetry, only one half of the domain is considered. Results have been validated with the experimental data for furnace temperature distribution. Effect of variation of parameters such as top air velocity, bottom air velocity, air temperature, furnace wall temperature and mass flow rate of coal are discussed for all three different types of coal. The effect of these various parameters has been discussed upon peak temperature inside the furnace, heat transfer to/from the system to surroundings and emission of gases like compounds of NO, CO and CO2. The analysis has been carried out using Ansys-Fluent software.

A numerical study of accelerated moderate or intense low-oxygen dilution (MILD) combustion stability for methane in a lab-scale furnace by off-stoichiometric combustion technology

Moderate or intense low-oxygen dilution (MILD) combustion has become a promising low-NO X emission technology, while the delayed mixing of reactants and slower oxidation rate could potentially cause ignition instability in some scenarios. This paper proposes a new idea for enhancing the ignition stability for methane MILD combustion by combining with off-stoichiometric combustion (OSC), and its performances have been numerically assessed through a comparison against the original MILD combustion burner. The results reveal although non-premixed pattern has the lowest NO emission, it suffers from a larger liftoff distance, thus less ignition stability. Contrarily, both partially-premixed and fully premixed patterns exhibit excellent ignition stability. Among the considered OSC conditions, the pattern of Inner ultra-rich and Outer lean produces the lowest NO emission while maintains a high ignition stability. Furthermore, the enhancement of the combustion stability by implementing OSC to the original MILD combustion burner is shown by comparing the operational range of furnace wall temperature ( T f ), CO and NO emissions, as well as the evolution of chemical flame. The comparison reveals that OSC can extend the lowest operational T f from 900 K to 800 K. More importantly, OSC can significantly improve the ignition stability in the whole range of T f as compared to the original MILD combustion burner.

Numerical analysis and modified thermodynamic calculation methods for the furnace in the 1000 MW supercritical CO2 coal-fired boiler

Supercritical carbon dioxide (S–CO2) coal-fired power generation technology is considered as the transformative technology in the energy sector due to its high efficiency, simple turbomachinery and high power density though the concept design of S–CO2 boiler is still in the infancy stage. This study constructs a modified combustion and heat transfer thermodynamic calculation method of the S–CO2 boiler adapted for multi-zone furnace. The effects of parameters and configurations on the characteristics of the S–CO2 boiler are compared and analyzed. The results show that the increase in the separated over-fired air ratio can effectively equalize the heat duty distribution. And the increasing of the furnace wall temperature will affect the furnace radiation heat transfer performance, which increases the flue gas average temperature. The boiler configurations including the flue gas recirculation, double furnace strategy, thermal insulation strategy and boiler local expansion strategy have the positive effects on decreasing the furnace heat duty to ensure the boiler safety. Finally, the novel S–CO2 boiler configurations are proposed and the comprehensive evaluations are conducted on the boiler combustion, heat transfer and burnout characteristics, which show that the novel S–CO2 boiler performance is promising and can provide a basis for the design of S–CO2 boiler.

Soft Sensor Modeling of Blast Furnace Wall Temperature Based on Temporal–Spatial Dimensional Finite-Element Extrapolation

The blast furnace (BF) wall temperature field plays a key role in determining whether the operation of the BF is efficient. However, it cannot be measured directly because the BF is an enclosed complex reactor with an extremely harsh environment. To cope with this problem, a soft sensor modeling strategy of the BF wall temperature field based on a novel temporal–spatial dimensional finite-element extrapolation algorithm (TS-FEEA) is designed. A 3-D transient heat conduction model of the BF wall is first established, which considers the layered physical structure of the BF. The large volume of the BF wall causes great calculation burden when solving the 3-D transient heat conduction model of the BF wall, to solve this problem, a TS-FEEA is proposed that realizes a high-speed and accurate model numerical solution. To further improve the 3-D model precision, the measured thermocouple temperatures in the cooling stave are served as the judgment criteria, and a correction strategy for inner wall temperature is finally designed to correct the assumed inner wall temperature through multiple iterations. Simulation results demonstrate the validity of the soft sensor modeling strategy and its applicability in industrial plants. The research achievements provide valuable reference information for the BF operators to judge the furnace conditions and adjust the operation parameters.

Exhaust temperature and peltier element optimization of thermoelectric generator output

The purpose of this study are to obtain (1) the optimum exhaust temperature for the highest voltage output, (2) the optimum number of peltier elements for the highest voltage output, (3) the optimum peltier circuit type for the highest voltage output. The analysis were done to obtain the optimum Seebeck coefficient as a representation of the thermoelectric generator performance. The design of this study is an L20 orthogonal matrix with three replications using a factorial design. The independent variables varied were exhaust temperature and number of peltier elements, while data analysis using ANOVA Two Way. Based on the results of the research data it can be concluded that the furnace wall temperature affects the TEG current output. The optimal wall temperature was 480 °C that produces a TEG current output of 168 mA. This indicates an increase in the temperature of the used metal smelting furnace walls cause the increase in the TEG current output. Meanwhile, the number of peltier elements influenced the TEG current output. The optimum values combination of exhaust temperature and peltier’s number which cause the highest TEG output voltage are 480 °C and 6 (six) peltier element in series circuit, respectively.

Characteristics of Nitric-Oxide Emissions from Traditional Flame and MILD Combustion Operating in a Laboratory-Scale Furnace

This study investigated the formation and emission characteristics of nitric oxide (NO) from flameless MILD (moderate or intensive low-oxygen dilution) combustion (MILDC) versus traditional visible-flame combustion (TC) in a 30-kW furnace. Both combustion processes were experimentally operated successively in the same furnace, burning natural gas at a fixed rate of 19 kW and the equivalence ratio of 0.86. Numerical simulations of TC and MILDC were carried out to explain their distinction in the measured furnace temperature and exhaust NO emissions. Present measurements of the NO emission (XNO) versus a varying furnace wall temperature (Tw) have revealed, at the first time, that the relationship of XNO ∼ Tw was exponential in both TC and MILDC. By analyzing the simulated results, the average temperature over the reaction zone was identified to be the common characteristic temperature for scaling NO emissions of both cases. Moreover, relative to TC, MILDC had a fairly uniform temperature distribution and low peak temperature, thus reducing the NO emission by over 90%. The thermal-NO formation was found to contribute more than 70%–80% to the total XNO from TC while the N2O-intermediate route dominated the NO emission from MILDC.

DYNAMIC MODELING OF MICRO-CLIMATE CONDITIONS OF A GREENHOUSE COUPLED WITH COAL FIRED HOT-AIR FURNACE

ABSTRACT In this study, a thermal analysis of a greenhouse heated with a coal-fired furnace was done. A greenhouse in the Mediterranean basin of Turkey was modeled. Energy and mass balances were derived to estimate the inside air temperature, humidity ratio and soil surface temperature, and the coal consumption rate. A dynamic approach was taken into account to define the time-dependent changes of the parameters in the greenhouse during the coldest period of the region, January and February. The main heat loss of the greenhouse occurs through the wall, ceiling and ground by convection and conduction. This heat loss is 63% of the total daily heat gains of the greenhouse for the coldest day. Furnaces were designed to maintain the greenhouse inside temperature approximately at 15ºC and averagely 200 kg coal was consumed per day. The maximum coal consumption amount was 480 kg for the coldest day. A meteorological data based solar radiation model was used, including dry-bulb temperature, relative humidity, wind speed and cloud cover. The effect of the cloud cover on greenhouse heating was investigated. For the coldest day, rather the sunrise was at 07:00 am, the furnaces were fired up to 10:00 am because of low solar radiation at that cloudy sky condition period. During the daytime, solar energy storage in the soil led to an increase in soil temperature. Approximately 1.5ºC-2ºC temperature difference on the soil surface was computed before and after the daytime. Finally, thermal characteristics of the furnace such as flue gas temperature and flow rate, furnace wall temperature, required amount of air for the combustion process were determined.

Spatially resolved solid-phase temperature characterization in a sillimanite tube furnace using a broadband two-color ratio pyrometry.

Tube furnaces are heating devices used for the synthesis of inorganic and organic compounds. It is essential to predict the spatially resolved temperature of solid substances placed inside tube furnaces in contact with its walls for a fixed steady temperature of the furnace walls. This enables efficient study of transport phenomena and control of the fabrication process in the furnace. In this work, the two-color ratio pyrometry (TCRP) using a digital single lens reflex camera has been used for the temperature characterization of a stainless steel metal sheet placed at the center of a 1000 mm long tube furnace. Temperature was measured for furnace walls set between 1000 K and 1426 K. The TCRP technique accounted for intensity from the heated target over the broadband visible region. The camera was calibrated and tested for signal linearity in its color channels for a fixed source illumination. The technique yields a mean sheet temperature of 979.5 K ± ∼24% (attributed to camera noise and uncertainties in gray level intensity, calibration lamp output, and monochromator and photodetector efficiency) and 1391 K ± 6.7% for a furnace wall temperature of 1000 K and 1426 K, respectively. Experiments showed that the effect of distance between the target and the camera on temperature measurement was negligible. Emission spectroscopy in the vis-near-infrared region (650-1100 nm) was also performed to predict sheet temperature. It yields results within 4.5% of TCRP at low furnace temperature but deviates by about 8.6% for temperatures above 1150 K, most likely due to experimental errors in spectroscopy. Analytical heat balance on the sheet, IR imaging, and numerical simulations yield temperatures within 5% of TCRP. This work shows that the TCRP technique can be used for spatially resolved temperature measurements of metals in tube furnaces and can readily be extended to ceramics or other class of solid materials whose emissivity can be shown to be invariant with wavelength in the visible region.

Ultrasonic Waveguide-Based Multi-Level Temperature Sensor for Confined Space Measurements

This paper reports the feasibility of using an ultrasonic waveguide sensor for distributed temperature measurement in two different case studies, that is: 1) skin temperature of a solid structure (pipe) and 2) fluid (water). This technique improves upon the conventional multiple thermocouples approach for multi-level temperature measurements. The range of temperatures is from room temperature to maximum utility temperature for the two case studies (85 °C for water and 200 °C for pipe). Using the interaction of the propagating ultrasonic waves in a thin rodlike waveguide with intentionally designed geometric discontinuities (bends, axisymmetric, non-axisymmetric notches, and so on), the localized information on the temperature is extracted. An ultrasonic technique for accurate temperature measurement by tracking the time-of-flight of reflected guided wave modes from appropriately spaced notch reflectors is therefore proposed here, while using the reflection from a bend is used as a reference. Using a finite element model approach, the notch size and/or the bend radius were selected in order to reduce mode conversion effects as well as to obtain uniform amplitudes of the reflected signals from these designed discontinuities. The numerical results were experimentally validated for the L(0,1) wave mode using a stainless steel waveguide sensor. This paper is of interest to industrial applications including mould cooling jacket temperature monitoring during the steel manufacturing process as well as for furnace wall temperature measurements in petrochemical industries.

Study of dual-phase drive synchronization method and temperature measurement algorithm for measuring external surface temperatures of ethylene cracking furnace tubes

Currently, the manual method using hand-held infrared temperature measurement instruments for measuring temperatures on the external surfaces of ethylene cracking furnace tubes is highly subjective and is affected by a number of prominent issues, such as the high temperature working environments, which leads to low efficiency and poor measurement accuracy. Hence, an automatic temperature measurement system based on infrared light is designed and realized. In the system, a dual-phase drive synchronization method is proposed to rotate the thermodetector during horizontal movements, thus realizing automatic batch temperature measurements of the furnace tubes. Moreover, a temperature processing algorithm is developed to automatically identify furnace wall and tube surface temperatures, filter out abnormal temperatures and select only high-quality temperature measurements prior to calculating the final result. Real temperature measurement experiments demonstrated that the dual-phase drive temperature measurement system and temperature processing method are effective and efficient. Compared with the traditional manual way, temperatures obtained using the proposed system are more stable and accurate.

Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475 K, in six different mixtures of O2/N2/CO2/H2O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O2/N2/H2O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O2/N2/H2O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.

Numerical simulation of a metallic load in continuous movement during heating in an experimental vertical-type furnace

The present work shows the results of a numerical simulation, in the transient 3D state, of heating a block of stainless steel AISI 304 in continuous movement in the interior of an experimental vertical furnace. The model considers the heat transfer by radiation between the furnace walls and the surface of the block using the model P-1, implemented in CFD Fluent® commercial software. The thermal boundary conditions were experimentally determined by sectioning and recording the furnace wall temperatures at different heights. In turn, the conditions were implemented through a user-defined function. The displacement of the block was modeled using two methods: the field assignment method and the layering dynamic mesh method. The simulated thermal histories obtained with both methods were compared with the experimental thermal history of the block during continuous movement in the interior of the furnace. The results show that the layering dynamic mesh method predicts the thermal history of the block heating during continuous movement more accurately than the field assignment method, providing a viable alternative for the simulation of continuous processes at high temperatures, such as the process of reheating a steel billet.

Energy losses from the furnace chamber walls during heating and heat treatment of heavy forgings

The boundary condition models at the inner and outer surface of the furnace wall have been developed taking into account heat transfer due to radiation and convection. The furnace wall temperature has been calculated from the finite element solution to the transient heat conduction equation. The boundary condition model has been validated using measurements on the laboratory furnace. The heat losses from the chamber walls have been calculated for selected wall structures and modes of the furnace operation. The results of computations have shown essential differences in the energy losses. It has been shown that the total heat losses depend on the chamber wall insulation and on the furnace operation mode for the most types of insulations. The best insulation effect has given the chamber wall made of the two types of the mineral fiber slabs. For this wall the total energy losses below 100 MJ/m2 has been obtained over 8 days of the furnace operation. Further, low sensitivity to the furnace operation mode has been noticed. This wall insulation can be utilized for heating ingots to the high temperatures and for the heat treatment processes characterized by a long time of the furnace operation.

Combustion behaviors of wasted plastics coated around electric cables in electrically heated drop tube furnaces

For thermal recycle by wasted plastics coated around electric cables, ignition and combustion experiments of the pulverized samples were carried out, using two types of electrically heated drop tube furnace, in which the particles are pulsatively fed and continuously fed, respectively. The ignition phenomenon was defined as occurrence of luminous emission from a cloud of the particles fed in the former furnace. The wasted plastics coated around the electric cables mainly make of polyethylene with a little amount of carbon particles. The ignition characteristics of the pulverized plastic samples as well as graphite particles as a reference sample were studied, changing the particle diameters and the furnace wall temperatures. In the electrically heated drop tube furnace with the continuous feeding system, the wall temperature and stoichiometric combustion air ratio were varied to elucidate the fundamental combustion characteristics. As a result, the ignition temperature depended on the particle diameter. The plastics with the particle diameter ranged from 150 to 300 µm could ignite at the lowest furnace temperature. From the continuous combustion tests, CO and lower molecule hydrocarbons like CH4, C2H2 and C2H4 were produced in the upper region of furnace. Finally, all of them gradually burned out.

MILD combustion of natural gas using low preheating temperature air in an industrial furnace

Moderate or intense low oxygen dilution (MILD) combustion is a promising strategy for reducing pollutant emissions and enhancing thermal efficiency. For industrial scale furnaces, the MILD combustion characteristics using lowly preheated air still needs further investigation. In this paper, an experimental study is presented on natural gas MILD combustion in a 0.3MW industrial furnace using low temperature air (130°C). First, the performances of MILD combustion burner are evaluated with comparison to conventional burner in terms of flow dynamics, temperature distribution and species formation. Subsequently, the transition process during the switching operation from conventional combustion to MILD combustion is examined, regarding the final emission as well as the furnace wall temperature variation. In addition, the impacts of switching temperature on the stability of the transition process are analyzed. The results show that, the present burner succeeds in establishing MILD combustion using low temperature air (130°C) even with a swirling fuel nozzle, resulting in quite uniform temperature distribution and over 80% reduction of NO emission as compared to conventional combustion. However, the lowly preheated air suppresses the combustion rate in the upstream furnace, leading to a backward movement of the high temperature reacting region under MILD operation. Nevertheless, the average furnace temperature is improved as compared to conventional combustion. More importantly, higher switching temperature is helpful for stabilizing the transition process with easier established MILD combustion mode as well as reducing CO emission during transition process.

Experimental study of the effect of a radiant tube on the temperature distribution in a horizontal heating furnace

This article reports the experimental study of the heat transfer characteristics in a horizontal heating furnace that is extensively used in the petroleum industry of China. To avoid furnace burnout caused by an uneven furnace wall temperature, a straight radiant tube is proposed to be installed in the furnace, which transfers heat to the furnace wall mainly via a radiation pattern. Seven groups of measure points were selected to record the temperature data; each group consisted of two testing points at the top wall and middle wall. Two burners were investigated to compare the temperature distribution along the horizontal furnace wall. A non-uniformity coefficient ε of the wall temperature was used to quantify the temperature deviation. The experimental results show that the radiant tube can significantly reduce the temperature deviation on the furnace wall. The furnace wall with the SR100 burner has a generally larger ɛ than that with the HQ05 burner without the radiant tube. The SR100 burner has a larger overall decrease in ɛ than the HQ05 burner after the radiant tube is installed.

Quantitative measurement and indication of pulverized coal ignition temperatures in O2/CO2 environments

The method on quantitative determination of the ignition condition of pulverized coal particles in a laminar drop tube furnace (DTF) was proposed with the continuous measurement of the evolved oxygen concentration in flue gas at variable furnace wall temperatures. Then the method was adopted to quantitatively determine the coal ignition temperatures in different O2/CO2 and air environments for five coal ranks with the measurement of the separate axial temperature profiles in the furnace centerline with and without coal particles feeding. The result shows that the coal ignition temperature in 21%O2/79%CO2 environment measured by DTF method is higher than that in air environment for each coal rank. Moreover, the coal ignition temperature significantly decreases with the increasing oxygen partial pressure in O2/CO2 environments, and the coal ignition temperature in O2/CO2 environment can match that in air environment when the oxygen partial pressure is increased to the value ranging from 25.8% to 44.1%, which is relative to different coal ranks.